President Obama on Wednesday nominated former first daughter Caroline Kennedy as US ambassador to Japan, offering the most famous living member of a prominent American family a new role of service to country.

The White House notified the Senate of the nomination and was planning to announce it later Wednesday, two people aware of the decision told The Associated Press on a condition of anonymity since they were not authorized to reveal it publicly ahead of the announcement.

Kennedy, an attorney and bestselling book editor, is being rewarded for helping put Obama in the White House where her father served until his assassination 50 years ago. If confirmed, she would be the first woman in a post where many other prominent Americans have served to strengthen a vital Asian tie.

Kennedy helped propel Obama to the 2008 Democratic presidential nomination in a celebrated endorsement over Hillary Clinton – the only time she's endorsed a presidential candidate other than her uncle Ted Kennedy in 1980. She played a prominent role, particularly in courting female voters by headlining swing state events for Obama in both his presidential campaigns.

She was a co-chair of Obama's vice presidential search committee and in the 2012 race served as one of 35 national co-chairs of his re-election campaign. She called Obama "the kind of leader my father wrote about in 'Profiles in Courage'" during a prime-time speech at the 2012 Democratic national convention.

Japan is one of the United States' most important trading and military partners and accustomed since the end of second world war to having renowned American political leaders serve as envoy. Former US ambassadors to Japan include former vice-president Walter Mondale, former House speaker Tom Foley and former Senate majority leaders Mike Mansfield and Howard Baker.

Kennedy doesn't have any obvious ties to Japan, a key ally in dealing with North Korea's nuclear ambitions. She would replace John Roos, a wealthy former Silicon Valley lawyer and top Obama campaign fundraiser.

She also would bring a third generation of her family into the US diplomatic corps. Her grandfather Joseph P Kennedy Sr was President Franklin D Roosevelt's ambassador to Britain, while her aunt Jean Kennedy Smith was ambassador to Ireland under Bill Clinton.

Caroline Kennedy was five days shy of her sixth birthday when her father was killed, and she lived most of the rest of her life in New York City. She married exhibit designer Edwin Schlossberg, and they have three children. She has served on the boards of numerous non-profit organizations, helped raise millions of dollars for New York schools and edited numerous bestselling books on history, law and poetry.

She considered running for political office after Clinton resigned the New York Senate seat to serve as Obama's secretary of state. But Kennedy eventually withdrew herself from consideration to fill the seat, once held by her uncle Robert F Kennedy, citing unspecified personal reasons.